Mesophase based carbon fibers are well known in the art since the issuance of U.S. Pat. No. 4,005,183. Numerous patents have issued relating the manufacture of mesophase pitch suitable for producing carbon fibers. Such patents include U.S. Pat. No. 4,026,788, U.S. Pat. No. 3,976,729, and U.S. Pat. No. 4,303,631.
It has been found in the art that mesophase pitch suitable for spinning pitch fibers contains at least 40% by weight mesophase so that the mesophase is the continuous phase, and the mesophase pitch upon quiescent heating forms domains at least 200 microns in size.
The spinning of mesophase pitch into continuous pitch fibers for the manufacturing of carbon fibers is usually carried out with a spinning apparatus which spins hundreds of fibers simultaneously, usually from 1500 to 2000 pitch fibers simultaneously. The average diameter of the pitch fibers is about 13 microns. The pitch fibers, say 2000, are treated together in subsequent steps. A bundle of continuous fibers is commonly referred to as "yarn" in the art. The carbon fibers are usually produced, packaged for shipping, and used in composites as yarns. Such yarns are sometimes referred to as "carbon yarns".
As used herein, the term "yarn" is a plurality of continuous fibers spun and processed together and the terms "pitch yarn", "infusibilized yarn", "carbon yarn" and "graphite yarn" are used to refer to the yarn at various stages of the manufacturing process.
Generally, the method for producing carbon fibers from mesophase pitch includes the steps of spinning the mesophase pitch into a plurality of pitch fibers (pitch yarn), infusibilizing the pitch fibers (infusibilized pitch yarn), and thereafter subjecting the infusibilized pitch fibers to a carbonizing step in a substantially non-reactive atmosphere for producing the carbon fibers (carbon yarn).
It is known from the prior art that the step of infusibilizing the pitch fibers is essential for the manufacture of carbon fibers because it enables the carbonizing step to be carried out relatively rapidly. The carbonizing step usually requires the yarn to be raised to a temperature of at least about 1000.degree. C. It is desirable to be able to raise the temperature of the yarn from about room temperature to the final temperature, for example 1000.degree. C., in a short time without causing deformation of the fibers, fusion between fibers, or a deterioration of the mechanical properties of the carbon yarn.
In the prior art, the infusibilizing step is particularly important for producing mesophase pitch based carbon fibers. Mesophase pitch derived carbon fibers are characterized by superior mechanical properties such as tensile strength and Young's modulus because the aromatic molecules of the mesophas pitch tend to orient parallel to the pitch fiber during the spinning of the mesophase pitch fibers. Raising the temperature of mesophase pitch fibers which have not been infusibilized to the softening point of the pitch fibers can result in the disorientation of the aromatic molecules and thereby substantially destroy the possibility of obtaining carbon fibers with superior mechanical properties.
The prior art has stressed the necessity of infusibilizing mesophase pitch yarn prior to the carbonizing step in order to avoid an extraordinary long period of time to raise the temperature of the yarn up from room temperature to the carbonizing temperature without deteriorating the qualities of the carbon yarn to be produced.
It is also essential, according to the prior art, to infusibilize non-mesophase pitch fibers to avoid having the fibers soften and thereby result in fusion between fibers in a yarn.
The step of infusibilizing pitch yarn is also referred to in the art as a "thermosetting step". The infusibilizing step is an exothermic reaction and the heat generated by the reaction can soften or deform fibers. The heat can cause fibers in a yarn to adhere or stick to each other and this reduces the tensile strength of the resulting carbon yarn as well as the properties of a composite made with the carbon yarn. This problem has been considered in U.S. Pat. No. 4,275,051 and U.S. Pat. No. 4,276,278.
The manufacturing of carbon fibers as reflected in the patent literature has been reviewed in the book entitled, "Carbon and Graphite Fibers, Manufacture and Application," published by Noyes Data Corporation, Park Ridge, N.J., 1980, edited by Marshall Sittig. This book sets forth the historical development of carbon fibers as derived from different precursor materials and the techniques patented for their manufacture. In addition, the book describes succinctly the various fiber treatment processes, matrices which are employed with carbon yarn in order to make composites, other reinforced materials which can be included in combination with carbon fibers to make effective composites, and the utilization of the carbon fibers in the manufacture of textile structures.
The International Committee for Characterization and Terminology of Carbon has published "First Publication of 30 Tentative Definitions" in Carbon, Vol. 20, pp. 445-449, 1982, to clarify the definition of many terms used in the art. The International Committee has defined "carbon fiber" as "filaments consisting of Non-Graphite Carbon obtained by Carbonization either of organic synthetic or natural fibres (PAN or others) or of fibres drawn from organic precursors such as resins or pitches, and by subsequent heat treatment of the carbonized fibres (up to temperatures of about 3000 K.)". The International Committee has also defined "Non-Graphitic Carbon" as "all varieties of substances consisting mainly of the element Carbon with two-dimensional long range order of the carbon atoms in planar hexagonal networks, but without any measurable crystallographic order in the third direction (c-direction) apart from more or less parallel stacking". The term "graphitic fiber" has been used in the art to describe carbon fibers which have been heat treated to between 2500 and 3000 K. The International Committee has pointed out that such fibers in most cases remain non-graphitic carbon so that the common term "graphitic fiber" is incorrect. The International Committe has pointed out, however, that "the term graphitic carbon is justified if Three Dimensional Crystalline Long Range Order can be detected in the material by diffraction methods, independent of the volume fraction and the homogeneity of distribution of such crystalline domains".
According to the prior art, the influsibilizing step is carried out in an oxidizing environment preferably at an elevated temperature in order to increase the rate at which the fibers become infusibilized. U.S. Pat. No. 4,389,387 discloses the problems of infusibilizing pitch fibers rapidly and effectively. The patent discloses that it is preferable to combine tens of thousands of pitch fibers into a tow of 10 to 30 mm in diameter in advance of the treatment for infusibilizing. The pitch fibers are loaded onto a net-belt conveyor and passed through a gaseous mixture of air and a gaseous oxidant such as oxygen, ozone, sulfur dioxide, nitrogen dioxide, etc. with the gaseous oxidant being 0.1 to 10% by volume of the gas mixture. The temperature for the infusibilizing step in the patent is lower than the softening point of the pitch fibers by at least 5.degree. C. to 50.degree. C. The time for infusibilizing is disclosed in the patent as from 1 to 4 hours. The patent states that problems of infusibilizing pitch fibers are overcome by moving the gaseous mixture through the packed pitch fibers. Nevertheless, the patent cautions against too large a packing height of pitch fibers to avoid insufficient removal of the generated heat.
South African Patent Application No. 71/7853, filed Nov. 4, 1971, entitled "Improvements In Or Relating To The Manufacture Of Carbon Fibers", disclosed processes for infusibilizing a fiber after it has been spun and prior to a carbonizing step. The infusibilizing step in the patent is referred to as "stabilizing". That is, "stabilizing" and infusibilizing are the same and are used interchangably in the patent. The precursor materials disclosed in the patent include solutions or extracts of coal, as well as pitches, pitch-like material and tar particularly if they are derived from coal.
The South African patent disclosed "that spun or extruded fibre, filament or film consisting of the organic material may be stabilized by heat treatment by reacting it with either an aqueous solution of bromine or an aqueous solution of nitric acid containing at least 25%, and preferably at least 40% by weight HNO.sub.3 for at least sufficient time to stabilize the spun or extruded fibre, filament or film to heat treatment". The patent further discloses that the stabilized fiber can be further stabilized for a heat treatment by oxidation employing an oxidizing gas, preferably containing molecular oxygen at an elevated temperature.
The South African patent discloses that nitric acid reacts with coal and similar materials decomposing the coal and that the reaction of the nitric acid with the coal is a surface effect, the nitric acid in certain circumstances reacting with the coal violently, or even explosively.
According to the South African patent:
"If the nitric acid is allowed to react for an excessive period of time with the spun or extruded fibre, filament or film of the organic material, the nitric acid may react with the spun or extruded fibre, filament or film of the organic material in such a manner as to cause it to decompose. In the case where the organic material is a solution or extract of coal as hereinbefore referred to, it is believed the nitric acid may react with the solution or extract of coal, cleaving the large molecules of the solution or extract of coal, thereby causing the solution or extract of coal to have smaller molecules. This might have the effect of diminishing the strength of a spun or extruded fibre, filament or film of the solution or extract of coal or of the carbon fibre, filament or film produced therefrom. Accordingly, the spun or extruded fibre, filament or film, whether of the solution or extract of coal or of other organic material, should not be allowed to react with the aqueous solution of either bromine or nitric acid for such a length of time as will seriously detrimentally affect the properties of the stabilized fibre, filment or film or the carbon fibre, filament or film produced therefrom." PA1 "The oxidation of filament wound to packages must follow a fairly critical heating regime if the superimposed and adjacent loops of filament are not to fuse together. This regime will naturally vary with the pitch, its previous oxidation history and the type and quality of additive present, if any. The best heating rates and soaking temperatures for a given material are naturally difficult to determine since the fusion temperature of the pitch changes as the oxidation proceed. Nevertheless, it has been established that a heat treated pitch of the type preferred, as described earlier, will yield filaments that are successfully oxidized by raising the temperature to 100.degree. C. in less than 15 minutes, a non-critical step; holding the filament at 100.degree. for about 20 hours; raising the temperatures from 100.degree. to 195.degree. C., at a preferred rate of about 5.degree. C./hour; holding the filament at the later temperature for a period within the range of about 60 to about 120 hours, the upper part of that range being preferred. It should be noted that with certain materials temperature increase rates of up to 10.degree. C./hour may be tolerated. In any event, the temperature at any time during the oxidation treatment should preferably be not higher than 10.degree. C. below the softening point of the pitch at the given time. This batch type oxidation is best carried out in a circulating oven through which passes a constant flow of air oxygen containing gas, both fresh and recycled, pre-heated at the desired temperature."
The South African patent provides a single example for the use of aqueous nitric acid. Example 1 discloses that a single filament having a diameter of 30 microns was cut into lengths and immersed in a solution comprising 50% by weight nitric acid at ambient temperature, about 20.degree. C. The number of cut lengths was not stated in the Patent. The fiber lengths was then washed with water to remove the nitric acid and suspended in a vertical oven which was heated in nitrogen to temperature about 260.degree. C. at a heating rate of 300.degree. C. per hour and thereafter, the nitrogen atmosphere was replaced by oxygen for five minutes. Subsequently, the fibers were heated in nitrogen at the rate of 80.degree. C. per hour to a temperature of 1000.degree. C. and this temperature was held for one hour.
The remaining two examples of the South African patent discloses the use of bromine in water instead of aqueous nitric acid. For each of these examples, the rate of temperature increase for the carbonizing step was 50.degree. C. per hour to a final temperature of 1000.degree. C.
The South African patent discloses that it is imperative that the nitric acid be washed from a fiber in order to avoid a deterioration of the fiber from the nitric acid. The commercial utilization of the disclosure of the South African patent would require a washing step subsequent to a nitric acid treatment and that subsequent to the nitric acid treatment, a heat treatment in oxygen similar to the aforementioned example 1 is necessary.
Significantly, each of the examples in the South African patents set forth a carbonizing treatment in which the temperature was increased to 1000.degree. C. at a rate of 50.degree. C. or 80.degree. C. per hour for separate cut lengths of the fiber suspended in a furnace. In contrast, a typically commercial carbonizing step for producing carbon fibers is for a yarn having typically at least 1000 filaments heated to a temperature of about 1000.degree. C. in a furnace through which yarn passes. The yarn is subjected to a change from room temperature to the carbonizing temperature and again to room temperature. The time the yarn is subjected to the carbonizing temperature is in the order of about one second or less.
Japanese Pat. No. 564,648, based upon Patent Publication No. 2510/69, published Feb. 3, 1969, discloses a process of producing carbon fibers from dry distilled petroleum sludge having a sulfuric acid content below 30%. Spun fibers are given a surface treatment by being exposed to chlorine gas stream at a temperature between room temperature and 60.degree. C. or dipped in a hydrogen peroxide, or hydrochloric acid, or nitric acid solution. Subsequently, the fibers are heated to 200.degree. C. or more in an oxidizing atmosphere to complete the infusibilizing step. The final step is a heat treatment for carbonizing the treated fibers to produce carbon fibers.
The Japanese patent discloses that the surface treatment is necessary because the direct heating in an oxidizing atmosphere of the spun petroleum sludge fibers results in the fibers becoming soft and deformed.
U.S. Pat. No. 3,595,946 discloses oxidizing treatments for filaments of pitch either continuously as the filaments are emerging from the spinning machine or for batches of filaments wound into packages. The hot filaments from the spinning machine are passed through an oxidizing atmosphere such as air, ozone, nitric oxide, etc. The patent discloses that the filament from the spinning machine can be cooled to a temperature below its fusion point and then passed through a liquid oxidizing bath such as nitric acid, sulfuric acid, chromic acid, permanganate solutions and the like. The patent discloses that the oxidizing treatments can be applied to batches of filament wound into packages. The patent cautions that "the support of the filament package must be of such nature and/or construction that it yields or collapses as the wound filament contracts during the oxidation process." The patent further cautions:
Such a heating schedule is extremely long in time even after tests have been carried out to optimize the process to avoid fusion between filaments.
In view of the prior art, it appears that it is essential to carry out a separate infusibilizing step prior to a carbonizing step and that considerable care must be taken for infusibilizing pitch yarn to avoid sticking or fusing of fibers. Many attempts have been made in the art to simplify and expedite the infusibilizing step. The art, however, does not disclose any process for infusibilizing yarn other than as a separate step.
Moreover, the prior art requires an oxidizing atmosphere to infusibilize pitch fibers even after the pitch fiber has been treated with an oxidizing liquid, such as nitric acid.
After carbon yarn has been produced according to the prior art, the carbon yarn must be cut into short lengths to be suitable for use in injection molding. The yarn lengths are about 6 mm and are often referred to as "chopped fibers" in the art.
Generally, the use of chopped fibers as well as chopped glass fibers and other materials with a matrix material for injection molding is well known. The chopped fibers can improve mechanical properties, electrical properties, and thermal properties of a mold object.
U.S. Pat. No. 4,032,607 discloses self-bonded webs of non-woven carbon fibers produced by spinning mesophase pitch fibers, disposing staple lengths of the pitch fibers in a intimately contacting relationship with each other in a non-woven fibrous web, heating the web in an oxidizing atmosphere for a time sufficient to thermoset the surfaces of the fibers of the web to an extent which will allow the fibers to maintain their shape upon heating to elevated temperatures but insufficient to thermoset the interior portions of the fibers, heating the fibers under compressive pressure in non-reactive atmosphere to cause the interior portions of the fibers to exude out and contact the surfaces of adjacent fibers, and further heating to elevated temperatures wherein fibers are bonded together by infusible carbon bonds.
For injection molding, a straightforward mixing of the chopped fibers with pellets of the matrix material while the mixture is fed into an injection molding apparatus has a serious drawback. The chopped fibers within a given length can become disassociated and form clumps of fibers which interfere with and disrupt the uniform feed into the apparatus. Such problems are avoided in the prior art by the use of a "master batch". A "master batch" is a bath of pellets containing a mixture of the matrix material and chopped fibers, usually about equal volumes.
The master batch is produced by mixing matrix material and chopped fibers and feeding the mixture into an extruder. The extruded material is cut into pellets. The chopped fibers can form clumps during the process of feeding the mixture into the extruder and this can interfere and disrupt the extrusion. This problem is minimized by the use of a size such as a phenolic binder on the thermoset yarn which is dried and chopped up. The chopped thermoset yarn is then collected in a sagger and carbonized. The size tends to retain fibers together with a chopped length and thereby inhibit the formation of clumps.
The web according to U.S. Pat. No. 4,032,607 is not suitable for injection molding because there is substantially no relatively free flowing properties needed for feeding an extruder or an injection molding apparatus. In any event, the patent disclosed infusibilizing at least partially by heating in an oxidizing atmosphere as a separate step.